1. Field of the Invention
The present invention relates to a glass plate provided with a conductive layer, which has a conductive layer formed on a glass plate of e.g. a rear window of an automobile to prevent fogging of such a rear window, a method for its production, a conductive paste which forms a glass exudate coating layer by firing, and a window for an automobile wherein the glass plate provided with a conductive layer is used.
2. Discussion of Background
To prevent fogging of a glass plate used for a rear window of an automobile, a plurality of conductive strips for heating and common power source lines (bus bars) connected to both ends of the conductive strips, are printed and fired on the glass plate. Metallic terminals for lead wires are attached to the bus bars by soldering. A voltage is applied across the metallic terminals to let the conductive strips generate heat so that the surface temperature of the glass plate is maintained at a level higher than the dew point, whereby fogging of the glass plate can be prevented.
The conductive strips are formed on the glass plate by printing a conductive paste on the glass plate and then firing it. In the case of a glass plate for an automobile, the width of such conductive strips is usually at most 1 mm and about 0.4 mm at the center portion of the glass plate, in order to secure the visibility.
Such a conductive paste is prepared usually by mixing a conductive material powder such as a silver powder, a low melting point glass powder, an organic varnish and in some cases, an inorganic refractory filler powder. When the printed conductive paste is fired, the conductive material powder such as a silver powder will be sintered to form a conductive layer. The surface of the conductive layer facing the substrate, is baked to the substrate by low melting point glass formed by melting of the low melting point glass powder. Here, the substrate means the glass plate or a ceramic color layer formed on the glass plate surface.
In recent years, along with the diversification of automobile designs, it is desired to make the rear window or the like provided with conductive strips slidable. If such a rear window or the like provided with conductive strips is made slidable, the conductive layer of the conductive strips is likely to be abraded due to friction between the rear window and the rubber packing, which takes place during the sliding operation. This is likely since the hardness of the conductive layer exposed on the surface of the conductive strips is small.
As a method for preventing such abrasion of the conductive layer, there has been known (1) a method wherein a dense and highly hard copper-nickel layer is formed by electroplating treatment on the surface of the conductive strips printed and fired on the glass plate, or (2) a method wherein a conductive paste to form conductive strips is coated with a predetermined width, then a glass paste is coated with a width wider than the above width, followed by heating for baking to form a glass layer on the surface of the conductive strips (JP-A-50-15514).
These conventional methods involve many process steps and accordingly have had problems such that the operation efficiency tends to be poor, and the cost tends to be high. Further, in the method (2), overflow of the glass layer beyond the conductive strips tends to be substantial, whereby a see-through image is likely to be deformed.